Project Summary/Abstract Telomeres, the specialized nucleoprotein structures located at the ends of eukaryotic chromosomes, are critical for genome stability. Telomere DNA, which consists of numerous copies of a short repeat, is difficult to maintain owing to (1) the end replication problem that prevents the complete duplication of parental DNA; and (2) the propensity of telomere DNA and chromatin to form replication barriers. The main players that help to overcome these difficulties include (1) telomerase, a special reverse transcriptase that adds ?G-strand? repeats onto the 3? ends of chromosomes; (2) primase-Pol a (PP), which adds ?C-strand? repeats onto the 5? ends of chromosomes; and (3) helicases and repair proteins that facilitate semi-conservative replication through telomeres. Telomerase has been subjected to detailed investigation and much is known about its mechanisms and regulation. Hence, in this application, we will focus on the roles of primase-Pol a and repair proteins such as Rad51 and Brh2 (BRCA2). The study will employ two fungal models (Candida glabrata and Ustilago maydis), each with its own unique advantages. In Aim 1, we will examine the mechanisms of PP and its regulation by CST, a telomere binding complex. We have identified a critical and conserved interface between the Stn1 and Pol12 subunits of CST and PP, and shown that this interaction likely triggers a conformational switch in PP to facilitate DNA synthesis. We will address this novel conformational switch mechanism using a combination of biochemistry, cyroEM and smFRET. In addition, both CST and PP have been linked to telomere replication and genome-wide replication stress response, though the underlying mechanisms are poorly understood. Accordingly, we will dissect the role of the CST-PP interaction in these pathways. These studies will be conducted using C. glabrata proteins because they are easily purified and biochemically tractable. In Aim 2 ? 3, we will address the mechanisms of two core repair proteins (Rad51 and Brh2[BRCA2]) in telomere replication and telomere capping. we have developed a high- resolution assay for telomere replication defects and used the assay to demonstrate critical functions for several repair proteins. We have also uncovered a novel and conserved interaction between Rad51 the telomere protein Pot1, which suggests novel, telomere-specific regulatory mechanisms. Hence in these two aims, we will dissect the mechanisms of Rad51 at telomeres and determine how its functions are regulated by Pot1 and Brh2 using a combination of genetics and biochemistry. Because RAD51 and BRCA2 factors have also been implicated in promoting replication and stabilizing stalled forks throughout the genome, our work may lead to a more integrated view of their mechanisms. This investigation will be carried out using Ustilago maydis because unlike standard fungi, U. maydis exhibits a high degree of similarity to mammals with respect to the recombination and telomere machinery.